1. Field
The present disclosure relates generally to aircraft and in particular to reducing electromagnetic effects on components in an aircraft. Still more particularly, the present disclosure relates to a method and apparatus for reducing electromagnetic effects from lightning strikes to aircraft structures containing composite materials and metal components.
2. Background
Aircraft are being designed and manufactured with greater and greater percentages of composite materials. Some aircraft may have more than fifty percent of its primary structure made from composite materials. Composite materials are used in aircraft to decrease the weight of the aircraft. This decreased weight improves performance features, such as payload capacities and fuel efficiencies. Further, composite materials provide longer service life for various components in an aircraft.
Composite materials are tough, light-weight materials, created by combining two or more dissimilar components. For example, a composite may include fibers and resins. The fibers and resins are combined and cured to form a composite material.
Further, by using composite materials, portions of an aircraft may be created in larger pieces or sections. For example, a fuselage in an aircraft may be created in cylindrical sections that may be put together to form the fuselage of the aircraft. Other examples include, without limitation, wing sections joined to form a wing or stabilizer sections joined to form a stabilizer.
In particular, carbon fiber reinforced plastic (CFRP) materials are examples of composite materials that are increasingly used for structural components in commercial aircraft in place of traditional aluminum structures. These types of composite materials are used because these materials provide a higher strength-to-weight ratio than aluminum.
Lightning strikes occur regularly on aircraft traveling near or through a thunderstorm. A lightning strike on an aluminum aircraft typically does not result in damage that affects the flight safety of the aircraft due to the material's highly effective ability to conduct and disperse the lightning current away from the point of attachment, but may leave a burn mark. Additionally, pits or burns on the aircraft may occur at the point of entry and/or exit for the lightning strike. In particular, a lightning strike arcing may occur between a fastener and a hole in the structure, in which fasteners are used to hold different structural components of the aircraft together. This type of arcing may induce a defect on the surface, which is also referred to as “pitting”.
Carbon fiber, however, is approximately two thousand times more resistive than aluminum and exists in a dielectric polymeric matrix. As a result, the amount of damage and possible sparking, at the skin of an aircraft where fasteners are, to the exposed surface may be higher than aluminum skins.
Damage to an unprotected carbon fiber reinforced plastic materials on an aircraft is often more severe than as compared to an aluminum structure. The temperatures caused by a lightning strike may heat up in carbon fibers, which have a lower thermal conductivity than metal. If the fiber temperatures in these materials exceed the pyrolyzation temperature of a surrounding matrix, the resin will transform from a solid to a heated gas. This pressurized gas may lead to delamination, punctures to the composite skin structures, and possibly the ejection of hot particles or sparks from the fastener interfaces and carbon fiber reinforced plastic joints.
These types of situations are currently prevented through various mechanisms for reducing both lightning damage and the possibility of fuel ignition as required for airworthiness and economic structure repairs. Lightning damage may result in the puncture or delamination of plies within the composite material.
Lightning protection systems are used to prevent the effects of a lightning strike without affecting the safety of flight. These types of systems assure that no sparking occurs at structural joints, on fuel couplings, and on hydraulic couplings within the fuel tank, as the lightning travels from an entry point to an exit point on the aircraft.
A number of technologies are currently available to provide protection for these types of composite materials on aircraft. Certain technologies are offered for diversion of lightning current from the attachment point to reduce the current density. Among them, one type of system involves the use of a copper grid co-cured into the composite skin layup, while another involves the use of conductive appliqué or decals applied over fasteners.
The copper grid lightning protection system integrates copper foil into the composite laminate at fastening areas. The copper foil, in this type of technique, is added to the part lay-up of composite skin prior to curing. The copper foil contacts selected fasteners to permit current distribution between the fasteners and reduce the current that may enter the fasteners.
Similarly, the conductive appliqué or decal is designed to divert lightning away from skin fasteners, preventing internal arcing and sparking to minimize damage to carbon fiber reinforced plastic skins and substructures. This type of system applies strips of dielectric layers and conductive layers to the cured composite skin using a pressure sensitive adhesive after fastener installation. In this type of system, the strips for the dielectric layer have an adhesive backing, which is placed onto the surface of the skin. Then, different strips of conductive layers may be placed over the dielectric layer and other areas of the skin. These strips of conductive layers also have an adhesive backing on the back of the strips.